Hot runner nozzles are used in injection molding tools in order to supply a flowable mass, for instance a plastic melt, at a specifiable temperature under high pressure to a separable mold insert. They generally have a material tube with a flow channel that terminates in a nozzle orifice. On its end, the latter forms a nozzle output that opens via a gate aperture into the mold insert (mold cavity).
An electrical heating element is provided on the material tube so that the flowable mass does not cool too soon inside the flow channel. The electrical heating element has a sleeve-like support element and an electrical conductor that produces heat when an electrical voltage is applied or when electrical current flows through it.
As is disclosed for instance in DE 10 2006 049 669 A1, the electrical conductor may be made of a heating coil formed from a resistance wire. In contrast, DE 10 2006 049 667 A1 uses for the electrical conductor so-called thick film heating elements that are applied to the support element as heating conductor strips in the screen printing process. Generally provided between the support element and the electrical conductor is an insulation layer that is also applied as a thick film during the screen printing process. In order to be able to reduce even further the size of the heating element or hot runner nozzle, as in DE 199 41 038 A1, it is also possible to apply the insulation layer and the heating conductor strips directly to the exterior wall of the material tube.
In the following we will proceed as an example from a thick film heater on a cylindrical support element. However, the invention is not limited to this, but rather may be used in the same manner with other heating elements, as well.
In order to supply the thick film heater with energy, provided at each end of the heating conductor strip is a terminal contact, e.g. in the form of a connection lead or a connection pin (see WO 2005/053361 A1, DE 10 2008 004 526 A1, and DE 10 2008 015 376 A1). The heating conductor strip is connected via the terminal contact to the connection line of a current source, it being necessary for such an electrical connection to satisfy all of the following requirements:                it must be embodied detachable in order to be able to install the hot runner nozzle in the injection molding tool and uninstall it and/or in order to be able to exchange a heating element if there is a defect;        it must always produce an adequate and reliable electrical contact between the terminal contacts in the heating conductor strips and the connection line that is connected to the current source;        due to the high temperatures during injection molding, it must be temperature-resistant and withstand mechanical tensile loads that can occur when installing and uninstalling the hot runner nozzle and/or when exchanging the heating element and from vibrations.        
U.S. Pat. No. 4,486,650 B1 discloses a detachable plug-in connection between the terminal ends of a heating coil and the connection line for a current supply. The ends of the heating coil are provided with contact pins that are oriented radially outward and that are each arranged approximately in the center of a collar. The ends of the connection line are provided with sleeve-like plug-in contacts that are each disposed in a plug housing. These are inserted into the collars, producing the electrically conductive connection between the outer surface of the contact pins and the inner surface of the plug-in contacts. To prevent the plug housing from separating from the collar, a clamping apparatus in the form of a union nut is screwed onto the male thread of the collar. Therefore a separate plug-in contact is needed for each connection.
WO 2004 043 672 A1 discloses a heating element for placing on a tubular conduit or a hot runner nozzle, the heating element being provided with a conductor through which current flows and that can be connected to a current source via connection lines. The connection lines have an end contact piece that can be placed against a contact surface of the heating element. Furthermore, a clamping apparatus is provided by means of which the contact piece is clamped against the heating element, the contact piece being positioned loose against the contact surface of the heating element and the clamping force of the clamping apparatus acting essentially normally to the contact surface and clamping the contact piece against the contact surface. Loosening the clamping apparatus, which especially includes a sleeve that can be screwed out of the heating element, causes the contact piece to come away from the contact surface so that the heating element can be removed and exchanged. The clamping force produced by the clamping apparatus is created either by a spring or by a tensioning screw.
Such a solution suffers from the drawback that the electrical contact between the contact surfaces of the heating element and the contact pieces of the connection line depends on both the quality and cleanness of the surfaces to be contacted and also on the clamping force of the clamping apparatus. There may be contact problems if the contact surfaces are uneven or soiled or if the clamping force from the spring decreases. This generally leads to lost power inside the heating element. Furthermore, excess heat may develop in the area of the terminal contacts, which may lead to damage to the heating element. Another drawback is the space required, since for each terminal contact a separate sleeve is screwed into the housing of the injection molding nozzle. This make assembling the connection line on and disassembling the connection line from the heating element quite inconvenient, as well.